1. Field of the Invention
This invention relates in general to radiation spectrum analyzers and radiation image analyzers, and in particular, to radiation analyzers and encoders that use spatial modulation of radiation dispersed by wavelength.
2. Background of the Invention
Radiation spectral analysis is presently carried out in a number of ways. Dispersive and Fourier transform based analyzers are for high resolution and can be used for many different applications so that they are more versatile than existing application-specific instruments and procedures. While these analyzers offer superior spectral performance, they tend to be expensive, large, heavy and non-portable. For most applications, these instruments offer a spectral resolution that is largely unnecessary. Many analytical computations can be made using relatively few spectral measurements. The processing of the additional, unnecessary optical data reduces the speed and compromises the photometric accuracy of these instruments.
In contrast, a non-dispersive approach to spectral analysis employs a radiation source filtered by one or more bandpass to provide input to a specific analytical function. The bandpass filters are used to select one or more specific spectral components, which are characterized by a center wavelength and bandwidth. One of the principal advantages of the non-dispersive approach is the ability to individually specify the center wavelength and bandwidth of the bandpass filters to optimize the instrument for a particular application. However, if the analytical function requires a significant number of bandpass filters, the system's signal-to-noise ratio is reduced as the total energy measured in a given filter over time is inversely related to the number of filters. Furthermore, if a spectrum analyzer using this approach is configured for a first application, the filters used in the device may have to be replaced, or the number of filters changed, to adapt the analyzer to a second application. As a consequence, the non-dispersive approach has clear limitations in adaptability and in the number of spectral components that can be analyzed.
Another type of optical spectrum analyzer is the Hadamard spectrometer, which is best described as a hybrid between dispersive and non-dispersive instruments. The Hadamard spectrometer includes a spatial radiation modulator, comprising a disc made of an opaque material with slots therein that reflect or transmit radiation, where the slots have uniform transmittance or reflectance. A radiation beam is dispersed according to wavelength onto the disc and the slots are selectively spaced at different radii from the axis to form a number of different optical channels for detecting corresponding spectral components of the beam. The disc is rotated about the axis and the slots selectively encode the corresponding spectral components with a binary amplitude modulation. The encoded beam is then directed to a detector. To differentiate the intensity of the spectral component transmitted or reflected by one slot from that of another, the disc is sequentially stepped through a specific number of steps, each step comprising a binary pattern of open or closed optical channels that defines one equation in a system of simultaneous equations for the amplitudes of the spectral components. This set of simultaneous equations is then solved to yield the intensity for each channel prior to any specific analytical function.
Not only is this approach time consuming, it is prone to errors. For example, as a direct consequence of the binary encoding approach, there is no mechanism by which one can recover the actual signal levels if any one of the signal levels changes significantly over the period of rotation. It should be noted that the system of equation can be simplified if the slots are patterned such that the radiation is transmitted or blocked one spectral component at a time (e.g., a filter-wheel photometer). However, this approach changes the optical duty cycle of each of the spectral components from its optimum value of 50%, thereby degrading the signal-to-noise ratio. Finally, if a Hadamard analyzer is configured for a first application and the number of slots is changed to adapt the analyzer to a second application, the data acquisition and decoding algorithms must be changed as well. This significantly limits the instrument's adaptability.
None of the existing approaches is entirely satisfactory. Therefore, it is desirable to provide improved spectrum and image analyzers where the above-noted disadvantages are avoided or significantly diminished, and where the encoding, data acquisition and decoding are both generalized and significantly simplified.